Secondary battery having multilayer battery case and method of manufacturing the same

ABSTRACT

A secondary battery includes an electrode assembly having a positive electrode, a negative electrode, a separator interposed therebetween, a positive electrode tab, and a negative electrode tab; a cap assembly having a current interrupt device to interrupt current when pressure increases, a safety vent connected to an upper end of the current interrupt device, and a top cap with an upwardly protruding shape; and a battery case to receive the electrode assembly and the cap assembly therein. The battery case has an inner layer adjacent to the electrode assembly and an outer layer of a material exhibiting lower thermal conductivity than the inner layer. The inner layer and the outer layer having a welding portion, at which only the predetermined regions of the inner layer and the outer layer are fixed.

TECHNICAL FIELD

This application claims the benefit of priority to Korean PatentApplication No. 2019-0067867 filed on Jun. 10, 2019, the disclosure ofwhich is hereby incorporated by reference herein its entirety.

The present invention relates to a cylindrical secondary battery havinga multilayer battery case and a method of manufacturing the same, andmore particularly to a cylindrical secondary battery having a multilayerbattery case configured such that only a predetermined region of a sheetmade of different kinds of metals including a copper material and anickel material, is ultrasonically welded so as to be used as a sheetfor battery cases and such that a negative electrode tab is made ofcopper, which is the same kind of metal as an inner layer of the batterycase, whereby it is possible to secure reliability in welding and toprovide a high-output secondary battery, and a method of manufacturingthe same.

BACKGROUND ART

As mobile devices have been continuously developed and the demand formobile devices has increased, secondary batteries, which are capable ofbeing charged and discharged, have been used as energy sources forvarious mobile devices. In addition, secondary batteries have alsoattracted considerable attention as energy sources for electric vehiclesand hybrid electric vehicles, which have been presented as alternativesto existing gasoline and diesel vehicles using fossil fuels.

Based on the shape of a battery case, secondary batteries are classifiedinto a cylindrical battery having an electrode assembly mounted in acylindrical metal can, a prismatic battery having an electrode assemblymounted in a prismatic metal can, and a pouch-shaped battery having anelectrode assembly mounted in a pouch-shaped case made of an aluminumlaminate sheet.

The electrode assembly, which is mounted in the battery case, is a powergenerating element that is configured to have a structure including apositive electrode, a negative electrode, and a separator that isinterposed between the positive electrode and the negative electrode andthat can be charged and discharged. The electrode assembly is classifiedas a jelly-roll type electrode assembly, which is configured to have astructure in which a long sheet type positive electrode and a long sheettype negative electrode, to which active materials are applied, arewound in the state in which a separator is disposed between the positiveelectrode and the negative electrode, or a stacked type electrodeassembly, which is configured to have a structure in which a pluralityof positive electrodes having a predetermined size and a plurality ofnegative electrodes having a predetermined size are sequentially stackedin the state in which separators are disposed respectively between thepositive electrodes and the negative electrodes. The jelly-roll typeelectrode assembly has advantages in that it is easy to manufacture thejelly-roll type electrode assembly and in that the jelly-roll typeelectrode assembly has high energy density per weight.

Meanwhile, in such a secondary battery, a negative electrode tabextending from a negative electrode is welded to the lower surface ofthe inside of a battery case by resistance welding. Resistance weldingis generally performed by inserting a first welding rod into a core ofan electrode assembly so as to contact the negative electrode tab andapplying current to a second welding rod in the state in which thesecond welding rod is in tight contact with the outer surface of thelower end of the battery case at a position opposite the first weldingrod. In the case in which a plurality of conductors contacts each other,heat is generated due to contact resistance, and welding is performed bythe heat. A larger amount of heat is generated at the contact portion ascontact resistance increases. Contact resistance increases as thecontact area between the conductors decreases. The inner surface of thelower end of a battery case of a general cylindrical secondary batteryand a negative electrode tab of the secondary battery are smooth andflat, whereby contact resistance is low, and therefore weldability isreduced.

In connection therewith, Patent Document 1 discloses a cylindricalsecondary battery configured such that a projecting portion is formed ona lower surface plate of a battery case in order to increase contactresistance when a negative electrode tab comes into contact with thebattery case, whereby it is possible to easily weld the negativeelectrode tab, but has a shortcoming in that the negative electrode tabis deformed by the projecting portion located at the fused surfacebetween the lower surface plate of the battery case and the negativeelectrode tab, whereby a ripple phenomenon may occur.

In addition, although the negative electrode tab is formed so as to havean embossed structure in order to weld the negative electrode tab, whichis made of a metal having low resistance, whereby welding processabilityis improved, the amount of heat generated when the negative electrodetab made of the low-resistance material is welded to the battery case,which has relatively high resistance, is considerably small, whereby itis not possible to secure processability to a preferred level, and aplurality of secondary batteries having welding defects may bemanufactured.

Patent Document 2 discloses a secondary battery including a cladnegative electrode tab having a two-layer structure including nickel andcopper. Since the clad negative electrode tab having the two-layerstructure including nickel and copper has higher resistance than anegative electrode tab made of a copper material, however, the maximumallowable current and heat dissipation characteristics of the cladnegative electrode tab are deteriorated, whereby it is not possible toprovide a high-output secondary battery.

PRIOR ART DOCUMENT

(Patent Document 1) Korean Patent Application Publication No.2007-0082969

(Patent Document 2) Korean Patent Application Publication No.2009-0132494

DISCLOSURE Technical Problem

The present invention has been made in view of the above problems, andit is an object of the present invention to provide a cylindricalsecondary battery having excellent allowable current and heatdissipation characteristics, whereby the output of the cylindricalsecondary battery may be increased, and a method of manufacturing thesame.

It is another object of the present invention to provide a cylindricalsecondary battery that does not use an expensive material, such as acopper-nickel clad, whereby production cost of the cylindrical secondarybattery may be reduced, and a method of manufacturing the same.

It is a further object of the present invention to provide a cylindricalsecondary battery that uses a negative electrode tab and a battery casemade of the same kind of metal, whereby resistance welding may be easilyperformed and the defect rate of products may be reduced, and a methodof manufacturing the same.

Technical Solution

A cylindrical secondary battery having a multilayer battery caseaccording to the present invention provided to solve the above problemsincludes an electrode assembly 100 including a positive electrode 110, anegative electrode 120, a separator 130 interposed between the positiveelectrode 110 and the negative electrode 120, a positive electrode tab140 having one side connected to the positive electrode 110, and anegative electrode tab 150 having one side connected to the negativeelectrode 120; a cap assembly 200 including a current interrupt device210 configured to interrupt current when pressure in the batteryincreases, the current interrupt device being located at the upper partof the electrode assembly 100, a safety vent 230 connected to the upperend of the current interrupt device 210, and a top cap 250 formed in anupwardly protruding shape, the lower surface of the outercircumferential surface of the top cap being in contact with the uppersurface of the outer circumferential surface of the safety vent 230; anda cylindrical battery case 300 configured to receive the electrodeassembly 100 and the cap assembly 200 therein, wherein the cylindricalbattery case 300 includes an inner layer 310 disposed adjacent to theelectrode assembly 100 and an outer layer 320 made of a material thatexhibits lower thermal conductivity than the inner layer 310, the innerlayer 310 and the outer layer 320 being provided with a welding portion330, at which only predetermined regions of the inner layer and theouter layer are fixed by welding.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the inner layer 310 may be madeof copper, and the outer layer 320 may be a nickel or nickel-platedsteel sheet.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may beformed by ultrasonic welding.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may belocated at the lower surface of the battery case 300.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may beformed in a circular band shape smaller than the inner diameter of thebattery case 300.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may belocated at the side surface of the battery case 300.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may belocated at the upper surface of the battery case 300.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, the welding portion 330 may belocated at two or more of the lower surface, the side surface, and theupper surface of the battery case 300.

A method of manufacturing a cylindrical secondary battery having amultilayer battery case according to the present invention includes astep of overlapping a copper sheet and a nickel or nickel-plated steelsheet to prepare a stacked-layer sheet; a step of welding only apredetermined region of the stacked-layer sheet to form a weldingportion; a step of deep-drawing the stacked-layer sheet to prepare acylindrical battery case configured such that the lower surface and theside surface of the cylindrical battery case are bent at a predeterminedangle and such that the upper part of the cylindrical battery case isopen; a step of receiving an electrode assembly and a cap assembly inthe cylindrical battery case; and a step of fixing the upper part of thecylindrical battery case.

Also, in the method of manufacturing the cylindrical secondary batteryhaving the multilayer battery case according to the present invention,the welding may be ultrasonic welding.

Also, in the method of manufacturing the cylindrical secondary batteryhaving the multilayer battery case according to the present invention,the welding portion 330 may be formed so as to have the shape of acircular band.

Also, in the method of manufacturing the cylindrical secondary batteryhaving the multilayer battery case according to the present invention,the circular band may include two or more circular bands havingdifferent inner diameters on the basis of an imaginary identical centralpoint.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a first preferred embodiment of thepresent invention.

FIG. 2 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a second preferred embodiment ofthe present invention.

FIG. 3 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a third preferred embodiment of thepresent invention.

FIG. 4 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a fourth preferred embodiment ofthe present invention.

FIG. 5 is a conceptual view illustrating a method of welding amultilayer sheet for battery cases applied to the first embodiment ofthe present invention.

FIG. 6 is a conceptual view illustrating a method of welding amultilayer sheet for battery cases applied to the fourth embodiment ofthe present invention.

FIG. 7 is a conceptual view illustrating a process of manufacturing thecylindrical secondary battery having the multilayer battery caseaccording to the first preferred embodiment of the present invention.

BEST MODE

In the present application, it should be understood that the terms“comprises,” “has,” or “includes,” etc. specify the presence offeatures, integers, steps, operations, components, parts, orcombinations thereof described in the specification, but do not precludethe presence or addition of one or more other features, integers, steps,operations, components, parts, or combinations thereof.

In addition, the same reference numerals will be used throughout thedrawings to refer to parts that perform similar functions or operations.In the case in which one part is said to be connected to another part inthe specification, not only may the one part be directly connected tothe other part, but also, the one part may be indirectly connected tothe other part via a further part. In addition, that a certain elementis included does not mean that other elements are excluded, but meansthat the other elements may be further included unless mentionedotherwise.

Hereinafter, a cylindrical secondary battery having a multilayer batterycase according to the present invention and a method of manufacturingthe same will be described with reference to the accompanying drawings.

FIG. 1 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a first preferred embodiment of thepresent invention.

Referring to FIG. 1, the cylindrical secondary battery having themultilayer battery case according to the first embodiment of the presentinvention includes an electrode assembly 100, a cap assembly 200, and acylindrical battery case 300 configured to receive the same.

The electrode assembly 100 may be a jelly-roll type electrode assemblyconfigured to have a structure in which a long-sheet type positiveelectrode 110 and a long-sheet type negative electrode 120 are wound inthe state in which a separator 130 is interposed therebetween, a stackedtype electrode assembly including unit cells, each of which isconfigured to have a structure in which a rectangular positive electrode110 and a rectangular negative electrode 120 are stacked in the state inwhich a separator 130 is interposed therebetween, a stacked and foldedtype electrode assembly configured to have a structure in which the unitcells are wound using a long separation film, or a laminated and stackedtype electrode assembly configured to have a structure in which the unitcells are stacked in the state in which a separator is interposedtherebetween and are then attached to each other.

A positive electrode tab 140 attached to the upper end of the electrodeassembly 100 is electrically connected to the cap assembly 200, and anegative electrode tab 150 attached to the lower end of the electrodeassembly 100 is connected to the bottom of the battery case 300.

Here, the negative electrode tab 150 is preferably made of a coppermaterial that has low resistance and high maximum allowable current.

Meanwhile, an insulating member (not shown) may be located at the upperpart of the electrode assembly 100, and the insulating member (notshown) serves to insulate the electrode assembly 100 and the capassembly 200 from each other.

The cap assembly 200 is located at the upper part of the insulatingmember (not shown) so as to be electrically connected to the positiveelectrode tab 130 attached to the upper end of the electrode assembly100, and is coupled to the upper open end of the battery case 300 so asto seal the electrode assembly 100 received in the battery case 300.

Specifically, the cap assembly 200 is configured such that a currentinterrupt device 210, a current interrupt gasket 220, a safety vent 230,a PTC element 240, and a top cap 250 are sequentially stacked frombelow, and a crimping gasket 260 is located at the outer edges of thecurrent interrupt device 210 and the top cap 250.

The positive electrode tab 140 is connected to the current interruptdevice 210 at a predetermined position of the lower surface thereof.Although the current interrupt device 210 is shown as being flat in thefigure, the central part of the current interrupt device may be convexupwards.

The safety vent 230, which is formed such that the central part of thesafety vent protrudes downwards, is located at the upper part of thecurrent interrupt device 210. The safety vent 230 interrupts current anddischarges gas in the case in which the pressure in the batteryincreases, and is disposed such that one surface of the safety ventcontacts the PTC element 240 and the end surface of the edge of thesafety vent contacts the crimping gasket 260.

The current interrupt gasket 220 is located such that the state in whichthe current interrupt device 210 and the safety vent 230 areelectrically insulated from each other can be maintained, except fordownwardly protruding portions of the current interrupt device 210 andthe safety vent 230.

One surface of the edge of the positive temperature coefficient (PTC)element 240, the resistance of which increases to interrupt current whenthe temperature in the battery increases, abuts the safety vent 230, andthe other surface the edge of the PTC element abuts the inner surface ofthe edge of the top cap 250.

For a cylindrical secondary battery, gas is generated in the battery dueto various causes, such as external impact, whereby the pressure in thebattery increases, and therefore the battery may catch fire or explode.

The current interrupt device 210 and the safety vent 230 are provided inorder to easily guide the discharge of gas acting as a cause ofincreasing the pressure in the battery, when the pressure in the batteryreaches a predetermined pressure or higher, predetermined portions ofthe current interrupt device 210 and the safety vent 230 are broken toprevent explosion of the battery.

The top cap 250, which is located at the uppermost part, seals the upperopen end of the battery case 300, and forms a positive electrodeterminal.

In a general cylindrical secondary battery, a crimping process and abeading process are performed in order to fix the cap assembly 200.

The crimping gasket 260 is attached to the edges, i.e. the outercircumferential surfaces, of the current interrupt device 210, thesafety vent 230, the PTC element 240, and the top cap 250 in order toprevent the above unit components from being deformed or damaged at thetime of performing the crimping process and the beading process and toimprove tight contact between the current interrupt device 210 and thetop cap 250. Here, the material for the crimping gasket 260 is notparticularly restricted as long as the material exhibits predeterminedelasticity and durability, and may be, as an example, poly butyleneterephthalate (PBT), polyphenylene sulfide (PPS), or perfluoroalkoxy(PFA).

The battery case 300 receives the electrode assembly 100 and the capassembly 200 therein, and the negative electrode tab 150 extendsdownwards and is then connected to the bottom of the battery case 300such that the bottom of the main body of the battery case 300 acts as anegative electrode.

Here, the battery case 300 has a multilayer structure including an innerlayer 310 and an outer layer 320. Specifically, the inner layer 310 isconnected so as to directly contact the negative electrode tab 150, andthe outer layer 320 is located outside the inner layer 310.

The outer layer 320 is preferably made of a material that exhibits lowerthermal conductivity than the inner layer 310. As an example, the innerlayer 310 may be a sheet made of a copper material, and the outer layer320 may be a nickel or nickel-plated steel sheet.

A circular-band-shaped welding portion 330 is formed at the edge of thebottom surface of the battery case 300 in order to fix the inner layer310 and the outer layer 320 to each other such that the layers aresecurely electrically connected to each other.

That is, in the battery case 300 according to the first embodiment ofthe present invention, the inner layer 310 and the outer layer 320 areadhered and fixed to each other via only the welding portion 330 formedby ultrasonic welding.

Therefore, the present invention has advantages in that, when thenegative electrode tab 150 made of the copper material is adopted, themaximum allowable current and heat dissipation characteristics thereofare excellent, whereby it is possible to provide a high-output battery,the negative electrode tab 150 and the inner layer 310 have the samemelting point, since the negative electrode tab 150 and the inner layer310 are made of the same kind of metal, i.e. copper, whereby it ispossible to easily fix the negative electrode tab 150 to the inner layer310 by resistance welding, and it is not necessary to use acopper-nickel clad, which is expensive.

FIG. 2 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a second preferred embodiment ofthe present invention. This embodiment is different from the firstembodiment of FIG. 1 in connection with the position of the weldingportion 330. That is, in the second embodiment, the welding portion 330may be located at a predetermined region of the side surface of thebattery case 300, rather than the edge of the bottom surface of thebattery case.

FIG. 3 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a third preferred embodiment of thepresent invention. This embodiment is different from the firstembodiment of FIG. 1 in connection with the position of the weldingportion 330. That is, in the third embodiment, the welding portion 330may be located at a predetermined region of the upper surface of thebattery case 300, rather than the edge of the bottom surface of thebattery case.

FIG. 4 is a sectional view of a cylindrical secondary battery having amultilayer battery case according to a fourth preferred embodiment ofthe present invention. Although the welding portion 330 is located atthe edge of the bottom surface of the battery case in the firstembodiment of FIG. 1, the welding portion 330 may be further formed at apredetermined region of the upper surface of the battery case 300 in thefourth embodiment.

Of course, although not shown in the figures, welding portions 330 maybe formed at the edge of the bottom surface of the battery case 300 andthe side surface of the battery case, the side surface and the uppersurface of the battery case 300, or the edge of the bottom surface ofthe battery case 300 and the side surface and the upper surface of thebattery case.

Next, a method of welding a multilayer sheet for battery cases will bedescribed with reference to FIGS. 5 and 6.

FIG. 5 is a conceptual view illustrating a method of welding amultilayer sheet for battery cases applied to the first embodiment ofthe present invention, wherein FIG. 5(A) is a sectional view showing thestate in which a multilayer sheet is seated in an ultrasonic weldingapparatus. Specifically, an inner layer 310 and an outer layer 320 eachhaving a predetermined width and length are placed above a die 410 in anoverlapping state, and an ultrasonic horn 420 is located thereabove.

When an ultrasonic wave having a predetermined range is applied throughtips 421 together with pressing, the temperature of correspondingregions of the inner layer 310 and the outer layer 320 is increased byvibration-based frictional heat, whereby the regions of the inner layerand the outer layer are joined to each other. Such an ultrasonic weldingprinciple is well known, and therefore a detailed description thereofwill be omitted.

FIG. 5(B) is a view of the multilayer sheet when viewed from above,which is a view showing the position and shape of a welding portion 330formed by ultrasonic joining. That is, when welding is performed whilerotating the ultrasonic horn 420 or the stacked-layer sheet includingthe inner layer 310 and the outer layer 320, a circular-band-shapedwelding portion 330 having a predetermined inner diameter is formed,whereby the inner layer 310 and the outer layer 320 are welded to eachother.

FIG. 6 is a conceptual view illustrating a method of welding amultilayer sheet for battery cases applied to the fourth embodiment ofthe present invention. Welding is performed using an ultrasonic horn 420having four tips 421 spaced apart from each other by a predetermineddistance (see FIG. 6(A)), and when welding is performed while rotatingthe ultrasonic horn 420 or the stacked-layer sheet, it is possible toform two circular-band-shaped welding portions 330 having differentinner diameters on the basis of an imaginary identical central point(see FIG. 6(B)).

Of course, it is possible to form a plurality of welding portions havingpredetermined shapes using an ultrasonic horn 420 having a single tip421 while rotating the stacked-layer sheet or the ultrasonic horn 420,and it is also possible to form a welding portion 330 having any ofvarious shapes, rather than a circular band shape.

Subsequently, a method of manufacturing a cylindrical secondary batteryhaving a multilayer battery case according to the present invention willbe described. FIG. 7 is a conceptual view illustrating a process ofmanufacturing the cylindrical secondary battery having the multilayerbattery case according to the first preferred embodiment of the presentinvention.

The method of manufacturing the cylindrical secondary battery accordingto the present invention includes a step of preparing a stacked-layersheet, a step of welding only a predetermined region of thestacked-layer sheet, a step of deep-drawing the welded stacked-layersheet to prepare a cylindrical battery case having an open upper part, astep of receiving an electrode assembly and a cap assembly in thecylindrical battery case, and a step of fixing the upper part of thecylindrical battery case.

The steps of preparing a stacked-layer sheet and welding only apredetermined region of the stacked-layer sheet have been described withreference to FIG. 5, and therefore a further description thereof will beomitted.

In the step of preparing a cylindrical battery case, the stacked-layersheet having a welding portion 330 formed thereat is placed at the upperpart of a mold 510 having a predetermined inner diameter and height, thestacked-layer sheet is pressed using a punch 520 from above thestacked-layer sheet, i.e. the stacked-layer sheet is deep-drawn, inorder to form the stacked-layer sheet such that the upper part of thestacked-layer sheet is open while the lower surface and the side surfaceof the stacked-layer sheet are bent at a predetermined angle.

Here, the welding potion 330 is located in the vertical surface of themold 510 such that the welding potion 330 is located at the edge of thebottom surface of the cylindrical battery case. It is obvious that, inthe case in which the battery case according to the second embodiment isformed, the welding portion 330 of the stacked-layer sheet must belarger than the bottom surface of the mold 510 and the welding portion333 must be located so as to wrap the bottom surface of the mold 510.

An electrode assembly 100 and a cap assembly 200 are sequentiallyreceived in the battery case prepared as described above through theupper end opening thereof, and then a jig 530 is pushed to perform acrimping process and a beading process.

Of course, it is reasonable to further perform a resistance weldingprocess in order to connect a negative electrode tab 140 made of acopper material to an inner layer 310 of the battery case 300 made ofthe same material.

Although the specific details of the present invention have beendescribed in detail, those skilled in the art will appreciate that thedetailed description thereof discloses only preferred embodiments of thepresent invention and thus does not limit the scope of the presentinvention. Accordingly, those skilled in the art will appreciate thatvarious changes and modifications are possible, without departing fromthe category and the technical idea of the present invention, and itwill be obvious that such changes and modifications fall within thescope of the appended claims.

DESCRIPTION OF REFERENCE NUMERALS

-   -   100: Electrode assembly    -   110: Positive electrode 120: Negative electrode    -   130: Separator 140: Positive electrode tab    -   150: Negative electrode tab    -   200: Cap assembly    -   210: Current interrupt device 220: Current interrupt gasket    -   230: Safety vent 240: PTC element    -   250: Top cap 260: Crimping gasket    -   300: Battery case    -   310: Inner layer 320: Outer layer    -   330: Welding portion    -   410: Die 420: Ultrasonic horn    -   421: Tip    -   510: Mold 520: Punch    -   530: Jig

INDUSTRIAL APPLICABILITY

In a cylindrical secondary battery having a multilayer battery caseaccording to the present invention, both a negative electrode tab and aninner layer of the battery case are made of the same kind of material,such as a copper material, whereby it is possible to secure reliabilityin welding and thus to contribute to a reduction in defect rate ofproducts.

Also, in the cylindrical secondary battery having the multilayer batterycase according to the present invention, both the negative electrode taband the inner layer of the battery case are made of a copper materialthat exhibits excellent maximum allowable current and heat dissipationcharacteristics, whereby it is possible to improve heat diffusion andthus to provide a high-output secondary battery.

Furthermore, in the cylindrical secondary battery having the multilayerbattery case according to the present invention, only a predeterminedregion of a multilayer sheet made of a copper material and a nickelmaterial, which are different kinds of metals, is ultrasonically weldedso as to be used as a sheet for battery cases without using acopper-nickel clad, which is expensive, whereby it is possible to reducemanufacturing costs.

1. A secondary battery having a multilayer battery case, the secondarybattery comprising: an electrode assembly comprising a positiveelectrode, a negative electrode, a separator interposed between thepositive electrode and the negative electrode, a positive electrode tabhaving one side connected to the positive electrode, and a negativeelectrode tab having one side connected to the negative electrode; a capassembly comprising a current interrupt device configured to interruptcurrent when pressure in the secondary battery increases, the currentinterrupt device being located at an upper part of the electrodeassembly, a safety vent connected to an upper end of the currentinterrupt device, and a top cap formed in an upwardly protruding shape,a lower surface of an outer circumferential surface of the top cap beingin contact with an upper surface of an outer circumferential surface ofthe safety vent; and a battery case configured to receive the electrodeassembly and the cap assembly therein, wherein the battery casecomprises an inner layer disposed adjacent to the electrode assembly andan outer layer made of a material that exhibits lower thermalconductivity than the inner layer, the inner layer and the outer layerbeing provided with a welding portion, at which only the predeterminedregions of the inner layer and the outer layer are fixed by welding. 2.The secondary battery according to claim 1, wherein the inner layer ismade of copper, and the outer layer is a nickel or nickel-plated steelsheet.
 3. The secondary battery according to claim 1, wherein thewelding portion is formed by ultrasonic welding.
 4. The secondarybattery according to claim 3, wherein the welding portion is located ata lower surface of the battery case.
 5. The secondary battery accordingto claim 4, wherein the welding portion is formed in a circular bandshape smaller than an inner diameter of the battery case.
 6. Thesecondary battery according to claim 3, wherein the welding portion islocated at a side surface of the battery case.
 7. The secondary batteryaccording to claim 3, wherein the welding portion is located at an uppersurface of the battery case.
 8. The secondary battery according to claim3, wherein the welding portion is located at two or more of a lowersurface, a side surface, and an upper surface of the battery case.
 9. Amethod of manufacturing a secondary battery having a multilayer batterycase, the method comprising: overlapping a copper sheet and a nickel ornickel-plated steel sheet to prepare a stacked-layer sheet; welding onlya predetermined region of the stacked-layer sheet to form a weldingportion; deep-drawing the stacked-layer sheet to prepare a battery caseconfigured such that a lower surface and a side surface of the batterycase are bent at a predetermined angle and such that an upper part ofthe battery case is open; receiving an electrode assembly and a capassembly in the battery case; and fixing the upper part of the batterycase.
 10. The method according to claim 9, wherein the welding isultrasonic welding.
 11. The method according to claim 9, wherein thewelding portion is formed so as to have a shape of a circular band. 12.The method according to claim 11, wherein the circular band comprisestwo or more circular bands having different inner diameters on a basisof an imaginary identical central point.